1. The Field of the Invention
This invention concerns a method for measuring the quality of a saturated steam flow. In particular, it relates to a method and apparatus for determining the quality of wet steam used for enhanced recovery of petroleum products from a reservoir, which steam has relatively low quality due to the presence of water with the steam's vaporous component.
2. The Prior Art
Steam flooding has become an accepted practice for secondary recovery of petroleum products from marginal fields or heavy oil reservoirs that require a degree of stimulation to produce a satisfactory flow of crude petroleum. There is a need for a simple method and apparatus to determine the quality of saturated steam at the wellhead of an injection well sending steam to such reservoirs. Such a measurement, if simplified, would be particularly useful in determining the amount of heat which is applied to the underground reservoir by the injected steam.
The measurement or monitoring of steam quality is important since the steam's quality, and thereby its reservoir or formation heatup effect, directly affects the resulting production operations. Further, the quality of the steam which can be most economically injected into a particular substrate or reservoir is contingent on a number of circumstances. The latter include the depth of the reservoir and the anticipated prospects for extracting commercially justified amounts of hydrocarbon products therefrom.
In brief, it is desirable that the quality of steam, that is the mass of the steam vapor divided by the total mass, and the mass flow rate, which is injected into each injection well be altered or adjusted to a level of quality that best conforms to the condition of the formation penetrated by that well. Clearly the quality of the steam and the mass flow rate must be known before any alteration or adjustment can be made.
It is known that in order to be particularly effective in this type of enhanced oil recovery operation (EOR), the flow of injected steam must be monitored by use of metering means positioned in the steam-carrying line adjacent the wellhead. It can be appreciated that steam will normally leave the steam generator or source at a known quality, pressure and mass flow rate. As the pressurized steam flow progresses towards an injection well, however, the quality will usually be substantially decreased. A decrease in the quality can be based on such factors as the distance between the well and the source, the effectiveness of the pipe insulation and weather conditions including ambient temperature and wind velocity. It will further depend on the pipe layout including number and orientation of pipe Tees through which the steam has to travel prior to reaching the injection port or well because of phase separation that can occur in these pipe Tees.
It is important, therefore, as a matter of economic practicality that a flow monitoring and controlling means be instituted into the steam pipeline immediately upstream of each injection wellhead. In many steamflood operations, a choke mechanism is placed in the steam line to constrict the steam flow to thereby allow regulation of the mass flow rate of the steam which enters that particular well.
In my prior U.S. Pat. No. 4,836,032, I disclosed the use of an orifice plate in series with a critical flow choke to provide a method of measurement for both steam quality and mass flow rate. Either the orifice plate or the choke alone can be used to measure steam quality and mass flow rate. However, a mathematical expression for steam quality through both devices is obtained by solving an independent mass flow rate equation for each device, an equation for wet steam through the critical flow choke and an equation for wet steam through a sharp-edged orifice plate. The present invention is distinguished from my earlier invention by the fact that the earlier invention requires two measurements, namely pressure at the entrance to the flow choke and the differential pressure across the orifice plate. The calculation procedure of the present invention uses three measurements: (1) upstream pressure before the choke; (2) downstream pressure after the choke; and (3) a differential pressure across the orifice plate. Application of a critical flow choke before the orifice plate requires a constant enthalpy quality change relationship because of the large pressure drop through the choke (between measurement points 1 and 2 above) to achieve a critical flow.